1. Field of the Invention
The present invention relates to a heating device incorporated in e.g. a photocopier for fusing a transferred toner image onto recording paper. It also relates to a method of making such a heating device.
2. Description of the Related Art
Referring to FIGS. 11 and 12 of the accompanying drawings, a conventional heating device (called xe2x80x9cheaterxe2x80x9d below) may have the following structure. The heater 9, as best shown in FIG. 11, includes an elongated supporting base 90 upon which two heating elements 91, 92 are formed to extend longitudinally of the base 90. The heating elements 91, 92 are made by printing and baking an Agxe2x80x94Pd resistive material for example. Except for the ends 91a and 92a, the heating elements 91 and 92 are covered by a crystalline glass layer 93 and a noncrystalline glass layer 94, as shown in FIG. 12. The exposed ends 91a, 92a of the heating elements are connected to an alternator 95. Upon application of the driving voltage, the heating elements generate heat, as required.
In operation, as shown in FIG. 12, recoding paper 96 is held in sliding contact with the outer glass layer 94 by a platen roller 97, so that the transferred toner image is fused onto the recording paper due to the heat generated by the heater 9.
In order to achieve high-speed printing, the recording paper 96 should be quickly heated up to a temperature beyond the melting point of the toner (up to about 230xcx9c250xc2x0 C.) by the heater 9.
If the supporting base 90 has high thermal conductivity, the heat generated by the heating elements will readily be dissipated through the base 90. Accordingly, the paper-contacting portion of the outer glass layer 94 may be cooled rather quickly down to e.g. the room temperature after the fixing unit is switched into the ready mode, where the power supply to the heating elements is temporarily stopped. Due to this, it may take a long time for the paper-contacting portion of the glass layer 94 to be heated up again to the temperature required for fusing the toner image. Apparently, this is disadvantageous to achieving high-speed printing.
If the supporting base 90 has low thermal conductivity, on the other hand, an uneven temperature distribution will result in the base 90 upon application of the driving voltage to the heating elements 91, 92. As a result, the base 90, subjected to an unacceptably great thermal stress, will be cracked or more severely damaged.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a heater that is thermally durable and capable of exhibiting an immediate thermal response.
According to a first aspect of the present invention, there is provided a heater that includes: a supporting base that has a first surface and a second surface opposite to the first surface and has a predetermined thermal conductivity; a heating element formed on the first surface; and a heat conductor having a thermal conductivity greater than the thermal conductivity of the base.
With the use of a heat conductor, the heat diffusion characteristics of the heater is improved to the extent that the supporting base is not thermally damaged, or that the warm-up time of the heater can be shortened than is conventionally possible.
Preferably, the heat conductor may be provided on the side of the second surface or the first surface. Further, the heat conductor may be provided between the first surface and the heating element.
Preferably, the heater of the present invention may further comprise a glass layer interposed between the first surface and the heat conductor.
Preferably, the heater of the present invention may further comprise a heat conduction restrictor having a thermal conductivity lower than the thermal conductivity of the base, wherein the heat conductor is provided on the side of the first surface of the base.
Preferably, the base may be made of an insulating material including Al2O3, and the heat conductor may be made of an insulating material including one of SiC, AlN, Ag, Al, BN and WC. As another possible example, the base may be made of an insulating material including AlN, while the heat conductor may be made of an insulating material including SiC.
According to a second aspect of the present invention, there is provided a heater that comprises: a supporting base including a first surface and a second surface opposite to the first surface, wherein the base has a predetermined thermal conductivity; a heating element formed on the first surface; and a heat conduction restrictor provided on the side of the second surface and having a thermal conductivity lower than the thermal conductivity of the base.
According to a third aspect of the present invention, there is provided a heater that comprises: a supporting base including a first surface and a second surface opposite to the first surface; and a heating element formed on the first surface of the base. The base includes a first and a second heat conduction restrictors and a heat conductor interposed between the first and the second heat conduction restrictors. The heat conductor is greater in thermal conductivity than the heat conduction restrictors.
According to a fourth aspect of the present invention, there is provided a method of making a heater. The method comprises the steps of: preparing a supporting base including a first surface and a second surface opposite to the first surface, wherein the base has a predetermined thermal conductivity; forming a heating element on the first surface; and providing a heat conductor on the base, wherein the heat conductor has a predetermined thermal conductivity. The thermal conductivity of the heat conductor is made greater than the thermal conductivity of the base.
Preferably, the heat conductor may be formed by sputtering, spraying, plating or screen printing.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.